The present disclosure relates generally to power generation systems and, more specifically, to systems and methods for adjusting floor pressure levels of heat recovery steam generators (“HRSGs”) during combined cycle plant startup to improve the speed and efficiency of the combined cycle plant startup.
At least some known power generation systems are combined cycle plants that include a multi-pressure heat recovery steam generator (HRSG) that uses combustion exhaust gas to generate progressively lower pressure steam as the exhaust gas is cooled. Relatively high grade heat at an exhaust gas inlet to the HRSG is capable of generating relatively high pressure steam in a high pressure stage or section of the HRSG. After heat is removed from the exhaust gas in the high pressure stage, the exhaust gas is channeled to an intermediate pressure stage where the relatively cooler exhaust gas is capable of generating a relatively lower pressure or intermediate pressure steam. The exhaust gas is then channeled to a low pressure stage of the HRSG to generate a low pressure steam.
During a combined cycle plant startup, a pressure limit (“floor pressure”) value is typically required at each section of the HRSG for proper operation. Establishing operation at these floor pressure levels is a pre-condition for initiating steam turbine admission. To pressurize to the floor pressure, the HRSG is generally sealed (i.e., bypass valves are closed) during the initial portion of the gas turbine startup. As pressure builds the only steam flow is from condensation inside steam pipes and HRSG tubing. Significant steam flow only begins when floor pressure is reached and bypass valves begin opening to control to floor pressure. While initial pipe warming occurs as steam condenses, full temperature above saturation temperature can only be achieved after steam flow is established. Since uniform steam pipe pre-warming depends on the presence of steam flows, establishing steam flow sooner reduces the time required to pre-warm the steam pipes to a suitable level appropriate to initiate steam turbine operation. Known methods for reducing start times involve the use of mechanical means (e.g., using faster rotor acceleration rates or plant loading rates), or expensive equipment specifically installed to decrease metal warming times such as steam pre-warming lines or trace heating. In such known methods, significant physical or capital investment is required to enable such reduced start times.
Accordingly, systems and methods for enabling faster steam availability and faster initial warming of the steam piping that avoid incurring the costs associated with known systems and methods are desirable. The described invention concerns a means of controlling to that lower startup floor pressure at key operating points during a startup to provide such benefits.